Japanese Patent Laid-Open No. Hei. 3-79460 discloses an anti-skid control system for maintaining a slip ratio of a wheel in the vicinity of an optimum slip ratio where the coefficient of friction of the wheel with respect to a road surface (hereinafter referred to as a surface .mu.) is maximized by detecting the wheel speed and the body speed, and by boosting/reducing the brake hydraulic pressure of the wheel cylinder corresponding to the detected deviation (i.e., the slip amount).
When the control for boosting the hydraulic pressure of the wheel cylinder continues for more than a predetermined time, the system judges that the surface .mu. has shifted from low .mu. to high .mu. and sets the fluid pressure boosting rate to be large. Thereby, the system can boost the fluid pressure quickly to the pressure required for giving the optimum slip ratio without causing the G slip-out phenomenon even when the surface .mu. changes suddenly at the time of braking.
However, because the predetermined time which is compared with the continuous run time of the boost control is fixed, the timing for increasing the gradient of boost is always the same. Accordingly, the timing for increasing the gradient of boost cannot be changed based on the amount of depression of a brake pedal. That is, there is a possibility that the deceleration expected by the driver cannot be obtained because the timing for increasing the gradient of boost cannot be hastened when the amount of depression of the brake pedal is large as compared to the case when the brake pedal depression amount is small.
Further, the above-mentioned system is not able to detect an increase in the amount of depression of the brake pedal (i.e. , the so-called increased tread) and does not change the fluid pressure boosting rate to be large when the increased tread or depression takes place. Accordingly, there exists the possibility that the body deceleration as expected by the driver is not obtained when the increased brake pedal depression takes place.
Accordingly, a need exists for a brake system that is able to change the timing for increasing the gradient of boost corresponding to a tread-on or depression amount of the brake pedal in an anti-skid control system by judging that a road surface has shifted from low surface .mu. to high surface .mu. when the continuous run time of the boost control exceeds a preset time.
A need also exists for a brake system that is able to obtain a body deceleration as expected by the driver when the driver presses on the brake pedal increasingly under the pulse boost control.